The invention relates to a linear actuator incorporating a stepping motor having a rotor which comprises a rotor shaft and a rotor-magnet sleeve, the rotor-magnet sleeve surrounding to the rotor shaft and imparting rotation to the rotor shaft.
Such stepping motors are generally known (Valvo UB of Philips GmbH, type no. 9904 112 27101).
Stepping motors are used for rotary and linear movements. In stepping motors for linear movements, the customary rotor shaft is replaced by a rotor shaft with an internal helical thread which cooperates with a threaded spindle. During rotation of the rotor shaft the threaded spindle is then reciprocated axially. Such stepping motors for linear movements are commonly known as actuators.
Generally, stepping motors must be started in a specific initial position. This initial position is detected by external means such as optical switches, light barriers or magnetic indicators.
These detection methods cannot be employed in the type of linear actuator described above, because the indicators used do not produce a satisfactory response for a displacement of the order of magnitude of 0.05 mm per step. Therefore, it is not possible to obtain a well-defined initial position in the case of small displacements.
These linear actuators have another disadvantage. They cannot cooperate with a mechanical stop. If this is required, it is found that the rotor may jump back, for example two steps, from the mechanical stop, so that the rotor magnet gets out of step--that is, the rotor position does not correspond to the stop position and the initial position is not correct when the motor is started again.
Until now these properties have prohibited use of rotary-step-drive linear actuators which must cooperate with a specific stop and which can perform accurate steps starting from this step.